A significant problem in the operation of plasma burners with alternating current and three phase electric current is the development of parasitic electric arcs which burn in parallel to the main electric arc. These parasitic electric arcs are predominantly concentrated at the lower edge of the nozzle or burner shell and the outer regions of the nozzle endpiece and the end face of the burner. Parasitic electric arcs are not only detrimental to the stability of the arc column and hence the formation and stability of the plasma, but also affect the efficiency and economy of the plasma burner and any apparatus in which the plasma burner is parallel to a significant degree because of the excess utilization of energy. Parasitic arcs can also give rise to complete destruction of the plasma burner
In German Patent Document DE-PS 33 28 777, a system for reducing parasitic arcs is described whereby the annular passage between the electrode and the nozzle along the inner surface of the nozzle is provided with an electrically insulating coating In practice this has been found to provide only partial protection since parasitic arcs can find current flow paths externally of the insulating coating.
Another approach to the elimination of parasitic arcs is described in German Patent Document DE-PS 34 35 680. In this system, on the inner wall of a water cooled nozzle, an insulating part is provided to subdivide it into two separate wall portions. This insulating part extends over the entire cross section and electrically insulates the separated parts from one another.
The drawback of this system is that it is difficult to find a fully satisfactory insulating material which can tolerate the extremely high temperatures of the furnace atmosphere in the apparatus in which the plasma burner is used.
Other attempts have been made to eliminate plasma arcs by providing a groove in an end face of the nozzle and accommodating in this groove an insulating ring. The groove can be formed by the outer wall of a nozzle stub or end piece and can have a burner shell such that the burner shell at its end has a flange turned toward the axis of the burner forming a heat shield for the insulating member which is set back from it.
When a burner of this type is used in an atmosphere containing electrically conductive particles, for example metal particles or molten plant or foundry dust, the electrically conductive dust can collect on the cooled insulating member and form an electrically conductive bridge shortcircuiting between the nozzle stub and the burner shell and sustaining parasitic arcs at least along the outer edge of the end face of the burner shell.
In general, therefore, it can be said that prior techniques attempting to eliminate parasitic arcs from plasma burners have not been fully satisfactory.